In conventional radionuclide emission scintillation cameras, gamma rays from a radioactive tracer in a body impinge upon and are absorbed by a sodium iodide scintillator. Such cameras, their calibration, and position analysis utilizing those cameras, is set forth in U.S. Pat. Nos. 4,095,107 and 4,228,515. An absorbed gamma ray causes excitation in the scintillator, with subsequent decay by means of light emissions with a half-life of two hundred fifty nanoseconds. These emissions are referred to as a scintillation event. Light from a scintillation event is distributed to an array of photomultiplier cells which are mounted on the surface of the crystal scintillator with a transparent light window. The position of a scintillation event is then determined by the charges in the pulses produced by the photomultiplier cells as a result of the absorption of scintillation light. At high count rates, some of the randomly occurring pulses overlap during the time needed to sample photomultiplier charges. This occurrence is referred to as pulse pile-up. When a pile-up of two or more events is accepted as a single event, a single erroneous position will be computed by the scintillation camera somewhere between the actual positions of the two events.
Presently, pulse pile-up events may be detected and rejected by two means: temporal and energy. The temporal technique rejects a perceived event if two or more pulses are detected within a sufficiently small interval of time such that they can both contribute to the pulse charge measurement significantly. The energy technique simply rejects events in which the total of the photomultiplier cell outputs exceeds some limit. Thus if the sum of the charges in the pulses from the photomultiplier cells is more than a prescribed amount, it is assumed that the charge was acquired from more than one event, and the input is rejected and no position determination is made based upon it. Although these techniques work alone and in combination in most applications, they may not be effective at high count rates in the range of tens to hundreds of thousands of counts per second. Under those conditions, an appreciable amount of multiple events or pulse pile-ups, may pass through both the temporal and the energy detection devices.
The problem is most acute when gamma rays are subjected to scattering in larger bodies. The scattering gives rise to Compton scattered photons, which are lower in energy than the original gamma ray emission. When two or more events occur close in time, their lower-energy Compton scattered photons may easily pass through the temporal window and the energy window as well.